[R] seq(2,5,-2) not an error but NULL

[Duncan Murdoch]

On 3/27/2006 4:41 AM, Christian Hoffmann wrote:
> Hi,
> 
> This may belong more to r-develop, but general discussion may be useful 
> (for the how many-th time ?)
> 
> seq(2,5,-2)
> seq(5,2,2)
> 
> both result in
> 
> Error in seq.default(2, 5, -2) : wrong sign in 'by' argument
> 
> But often, if not always, mathematicians and programmers want a 
> behaviour e.g. in for loops, where this statement results in an empty 
> statement, that is
> 
> for (ii in seq(2,5,-2)) print(ii)
> 
> were equivalent to
> 
> for (ii in NULL) print(ii).
> 
> The relevant part in seq.default is now
> 
>              if (n < 0)
>                  stop("wrong sign in 'by' argument")
> 
> but could be changed by option to
> 
>    return(NULL)
> 
> I think there should be an option to seq requiring this behaviour, or a 
> specific function, may be even a special operator, e.g. %;%:
> 
> 3;5 resulting in NULL.
> 
> What do you think?

If you want optional behaviour, the easiest way is to write your own 
wrapper function.  E.g.

emptyseq <- function(from, to, by) {
   if ((to-from)*by < 0) return(NULL)
   else return(seq(from, to, by))
}

I don't think this is a desirable default, though.  We already have a 
special way to handle the most common case, i.e.

seq(1, length(x), 1)

should be written as

seq(along=x))

to handle the length(x) == 0 case the way you're requesting.

But I'm not so sure that seq(2,5,-2) should really be NULL; it looks 
much more like an error to me.  You say mathematicians and programmers 
want this behaviour, but I really can't think of any examples other than 
the one above.

As a general principle, I think it's better to throw an error on 
ambiguous or apparently erroneous code rather than always returning an 
answer.  If the code can be made unambiguous, it should be.  (R doesn't 
always follow this principle; for example, recycling of vectors of 
lengths bigger than 1 is probably an error at least as often as it's 
intended.)

Duncan Murdoch